1. Technical Field
The present invention relates generally to a reciprocating device and linear suspension and, more particularly, to a reciprocating device and linear suspension element for eliminating costly precision fasteners and corresponding precision holes when attaching a reciprocating device moving element and linear drive mechanism.
2. Related Art
There are a variety of linear suspensions available to constrain movement of a moving element in a linear motor or reciprocator. These suspensions are analogous to bearings in rotary devices because they restrict a moving element to primarily move in the operationally useful sense and prevent motion in other directions. Additional objectives of a linear suspension (as opposed to linear bearings) are to provide long-life reciprocation with no friction or wear and to eliminate wear at start-up and shut-down or low-stroke operations when normal bearings develop insufficient velocity or pressure to function.
U.S. Pat. No. 5,552,214 to Beckett et al. discloses an exemplary spiral suspension (sometimes referred to as an “Oxford” spring) for a reciprocator. This suspension spirally expands to allow its center to move perpendicularly to the spiral plane. It resists radial movement to limit linear motion of a moving element to a single axis perpendicular to the spiral plane.
A problem with spiral suspensions is their inducement of a torsional motion about the reciprocation axis. See U.S. Pat. No. 5,522,214 to Stirling Technology Corporation. This motion may cause vibration and failure and requires the moving element to be circular in section, or have excessive clearances to the stationary elements of an assembly in order not to collide with the stationary elements during operation. Accordingly, spiral suspensions are useless with reciprocators as disclosed in U.S. Pat. No. 5,389,844 to Yarr et al. (commonly called “STAR” reciprocators) in which the moving element and stationary elements mate in a generally non-circular manner. In these reciprocators, the suspension must exhibit substantial torsional stiffness as well as radial stiffness to prevent running contact between reciprocating/moving and stationary elements.
Strap suspensions, such as described incidentally in U.S. Pat. No. 5,389,844 to Yarr et al., have been developed for “STAR” reciprocators. In these suspensions, radial straps are provided and anchored at at least two points at each end to resist torsional movement and radial movement of the moving element. Tensile stresses experienced by the radial portions of the legs of the flexure element strap are transferred to bending stresses in the vertically mounted ends of the legs (oriented approximately 90 degrees to the radial portions). A potential problem with these suspensions, however, is that fretting may occur on the ends of the flexure elements where they are clamped, especially at large strokes and strap strains. Furthermore, the clamping of the legs to the mount may be mechanically cumbersome.
To utilize the torsional stiffness within a suspension, the mechanical connection of the suspension to the moving element and stator must also be resistant to torsional loads. Further, if the moving element-stator interface has been designed in a non-circular form, then there exists a preferred, or required, precise relative angular positioning between the moving element and stator where sufficient interface clearance exists to allow non-contacting movement of the moving element adjacent to the stator. To ensure such angular positioning, others have used precision pins or shouldered threaded fasteners to attach the suspension elements to the moving element and/or stator, thereby aligning them together in both centric and angular position. Such alignment features are expensive to add as they require high precision in both hole-making and in fastener dimensions, as well as precision in location of the aligning features with respect to the features to be aligned (e.g., magnets, stator pole faces, etc.).
Even designs that are circular in section at the interface between moving element and stator use and benefit from precision location features between these parts. Although rotation and angular position will not directly lead to contact between moving element and stator in these devices, eccentricity will, forcing at least one alignment feature to ensure concentricity. Further, the attachment of the suspension must not allow local relative motion between the suspension and its mounts, or fretting damage to that interface can occur, leading to premature failure of the suspension. This is especially true in the commonly-used spiral-planar suspensions because the operation of the springs necessarily imparts a jerky torque between stator and moving element by the twisting (and untwisting) required in this kind of suspension to accommodate reciprocation. This is true even when the mountings are improved as shown, for instance, in U.S. Pat. No. 6,050,556 to Masuda et al. Additional complexity is created where multiple suspension elements are fitted in parallel, and it becomes necessary to align the elements in both centric and angular manners. In devices like those of Redlich, U.S. Pat. No. 4,602,174, or Bhate, U.S. Pat. No. 4,349,757, the radial laminations of the stator make this an awkward and expensive mounting, as axial holes can only be provided in auxiliary rings separately attached to the radial laminates.
In view of the foregoing there is a need in the art for a suspension element capable of withstanding operational and non-axial stresses and providing longer life. In addition, there is a need in the art for a suspension element that allows for precise and inexpensive alignment of parts. Further, there is a need for a reciprocating device and suspension having the same characteristics.